1. Field of the Invention
The present invention relates to digital oscillators having a constant DC supply current.
2. Description of Related Art
FIG. 1 shows a conventional oscillator 10 including a differential op amp 11 connected to +5 and -5 volt supplies and having an output terminal V.sub.out. A positive feedback loop consisting of equal value resistors 12 and 13 is coupled between the non-inverting and output terminals of op amp 11, and a negative feedback loop consisting of resistor 14 and capacitor 15 is coupled between the inverting and output terminals of op amp 11.
When power is initially provided to oscillator 10, output V.sub.out will be at either the +5 or -5 volt rail. For the discussion that follows, we will assume that V.sub.out is initially at the +5 volt rail. A toggle voltage of approximately 2.5 volts will appear at the non-inverting terminal of op amp 11 via positive feedback resistors 12 and 13. Current flowing through resistor 14 will charge capacitor 15 towards the toggle voltage of 2.5 volts. As the inverting terminal of op amp 11 reaches 2.5 volts, V.sub.out will swing low to -5 volts.
Once V.sub.out toggles low to -5 volts, resistors 12 and 13 will pull the non-inverting terminal of op amp 11 down to approximately -2.5 volts. Capacitor 15 will then discharge and gradually pull the inverting terminal of op amp 10 to -2.5 volts. Just as the inverting terminal voltage reaches that of the non-inverting terminal, i.e., -2.5 volts, op amp 11 toggles V.sub.out high to 5 volts. In this manner, V.sub.out oscillates between 5 and -5 volts, where the oscillation frequency depends upon the RC time constant of the negative feedback loop.
Oscillator 10 has associated therewith several disadvantages. First, oscillator 10 draws an undesirably large amount of supply current from the 5 and -5 volt supplies, thereby resulting in an undesirable amount of power dissipation. Second, capacitor 15 is typically on the order of a few micro-Farads and, being too large to form as an on-chip component, must be formed as a discrete element. Such discrete capacitors are not only expensive but also result in a wasting of a large amount of board area.
Further, oscillator 10 is problematic for some mixed signal applications in which digital and analog components are formed on a common substrate. As the oscillating frequency increases, the slew rate of V.sub.out causes undesirable current spikes on the +5 and -5 volts supplies. These current spikes, commonly known as switching transients, may be coupled from oscillator 10 to the sensitive analog circuitry (not shown) via the common substrate. This digital to analog crosstalk phenomenon may perturb the operation of the sensitive analog circuitry. It would thus be desirable to reduce the effects of such digitally-generated switching transients upon the operation of the analog components.